Oximeter with motion detection for alarm modification

ABSTRACT

A pulse oximeter which modifies the alarm condition when motion is detected. Basically, if the lack of a pulse is determined to be as a result of motion artifact, the generation of an alarm is postponed. In addition, the display indicates that motion is present and that the last reading is questionable due to the presence of motion. The invention also determines if motion artifact is present from the pulse oximeter detector signal itself. The ratio of the positive and negative peaks of the derivative of the pulse signal are compared to a motion/blood pulse threshold.

This is a continuation of application Ser. No. 08/304,914, filed Sep.13, 1994, now abandoned, which is a continuation of application Ser. No.08/037,953, filed Mar. 26, 1993, now U.S. Pat. No. 5,368,026.

BACKGROUND

The present invention relates to a pulse oximeter for detecting bloodoxygenation, and in particular to the detection of motion artifact whichmay affect the detected blood oxygenation signal.

Pulse oximeters typically measure and display various blood flowcharacteristics including but not limited to blood oxygen saturation ofhemoglobin in arterial blood, volume of individual blood pulsationssupplying the flesh, and the rate of blood pulsations corresponding toeach heartbeat of the patient. The oximeters pass light through human oranimal body tissue where blood perfuses the tissue such as a finger, anear, the nasal septum or the scalp, and photoelectrically sense theabsorption of light in the tissue. The amount of light absorbed is thenused to calculate the amount of blood constituent being measured.

The light passed through the tissue is selected to be of one or morewavelengths that is absorbed by the blood in an amount representative ofthe amount of the blood constituent present in the blood. The amount oftransmitted light passed through the tissue will vary in accordance withthe changing amount of blood constituent in the tissue and the relatedlight absorption.

For example, the Nellcor N-100 oximeter is a microprocessor controlleddevice that measures oxygen saturation of hemoglobin using light fromtwo light emitting diodes (LED's), one having a discrete frequency ofabout 660 nanometers in the red light range and the other having adiscrete frequency of about 900-920 nanometers in the infrared range.The N-100 oximeter microprocessor uses a four-state clock to provide abipolar drive current for the two LED's so that a positive current pulsedrives the infrared LED and a negative current pulse drives the red LEDto illuminate alternately the two LED's so that the incident light willpass through, e.g., a fingertip, and the detected or transmitted lightwill be detected by a single photodetector. The clock uses a highstrobing rate to be easily distinguished from other light sources. Thephotodetector current changes in response to the red and infrared lighttransmitted in sequence and is converted to a voltage signal, amplified,and separated by a two-channel synchronous detector--one channel forprocessing the red light waveform and the other channel for processingthe infrared light waveform. The separated signals are filtered toremove the strobing frequency, electrical noise, and ambient noise andthen digitized by an analog to digital converter.

The detected digital optical signal is processed by the microprocessorof the N-100 oximeter to analyze and identify optical pulsescorresponding to arterial pulses and to develop a history as to pulseperiodicity, pulse shape, and determined oxygen saturation. The N-100oximeter microprocessor decides whether or not to accept a detectedpulse as corresponding to an arterial pulse by comparing the detectedpulse against the pulse history. To be accepted, a detected pulse mustmeet certain predetermined criteria, for example, the expected size ofthe pulse, when the pulse is expected to occur, and the expected ratioof the red light to infrared light of the detected optical pulse inaccordance with a desired degree of confidence. Identified individualoptical pulses accepted for processing are used to compute the oxygensaturation from the ratio of maximum and minimum pulse levels as seen bythe red wavelength compared to the maximum and minimum pulse levels asseen by the infrared wavelength.

The optical signal can be degraded by both noise and motion artifact.One source of noise is ambient light which reaches the light detector.Another source of noise would be electromagnetic coupling from otherelectronic instruments in the area. Motion of the patient can alsoaffect the signal. For instance, when moving, the coupling between thedetector and the skin or the emitter and the skin can be affected, suchas by the detector moving away from the skin temporarily, for instance.In addition, since blood is a fluid, it may not move at the same speedas the surrounding tissue, thus resulting in a momentary change involume at the point the oximeter probe is attached.

Such motion can degrade the signal a doctor is relying on, with thedoctor being unaware of it. This is especially true if there is remotemonitoring of the patient, the motion is too small to be observed, thedoctor is watching the instrument or other parts of the patient, and notthe sensor site, or in a fetus, where motion is hidden.

In one oximeter system described in U.S. Pat. No. 5,025,791, anaccelerometer is used to detect motion. When motion is detected,readings influenced by motion are either eliminated or indicated asbeing corrupted. In other systems, such as described in U.S. Pat. No.4,802,486, assigned to Nellcor, an EKG signal is monitored andcorrelated to the oximeter reading to provide synchronization to limitthe effect of noise and motion artifact pulses on the oximeter readings.This reduces the chances of the oximeter locking on to a periodic motionsignal. Still other systems, such as that set forth in U.S. Pat. No.5,078,136, assigned to Nellcor, use signal processing in an attempt tolimit the effect of noise and motion artifact. The '136 patent, forinstance, uses linear interpolation and rate of change techniques orselective frequency filtering to analyze the oximeter signal.

Many pulse oximeters have audible alarms which will activate if no pulsesignal is detected for a certain period of time, such as 10 seconds.This is clearly desirable to detect when a patient has lost his or herpulse. However, when noise or motion artifact corrupts the pulse signalsand prevents the detection of sufficient qualified pulses in a 10 secondperiod, false alarms can be frequently generated and are not only veryannoying, but can reduce the confidence in a true alarm situation.

SUMMARY OF THE INVENTION

The present invention provides a pulse oximeter which modifies the alarmcondition when motion is detected. Basically, if the lack of a qualifiedpulse is determined to be as a result of motion artifact, the generationof an alarm is postponed. In addition, the display indicates that motionis present and that the last reading is questionable due to the presenceof motion.

In a preferred embodiment, the oximeter operates in three differentstates. First, in a normal state, qualified pulses are present andprocessed, and blood oxygen and pulse readings are generated. Second, ina noise state, an alarm time-out period begins to run when there is anabsence of sufficient qualified pulses which are not due to motionartifact. Third, a motion state is entered when the lack of sufficientqualified pulses is determined to be due to the presence of motionartifact. This causes the alarm period to be extended.

In a preferred embodiment, when no qualified pulse has been detected for10 seconds, the oximeter enters a probationary state. A 6.3 second timeris set upon entering the probationary state. 6.3 seconds allows time for2 heart pulses at 20 beats/min., plus a 5% cushion. If it is determinedthat the oximeter is in a noise state during this period, an alarmsounds after 6.3 seconds. If, instead, it is determined that motion ispresent and the oximeter enters the motion state (motion artifact isdetected as causing pulses) the alarm generation is extended up to amaximum of 50 seconds. The oximeter can exit the motion state and enterthe noise state upon the cessation of the detection of pulses due to themotion artifact, in which case the 6.3 second timer will reset andrestart.

Finally, the motion state can be exited and a return to the normal statecan occur if a number of clean, qualified pulses are detected. This ispreferably the same number of pulses required to be detected uponstartup of the oximeter to establish a pulse reading and lock-on to apulse frequency, but the return criteria may be different from theinitial lock-on criteria. Thus, the exiting of the probationary state ismade difficult to ensure that a clean pulse signal is indeed present.

The invention also provides a method and apparatus for determining ifmotion artifact is present from the pulse oximeter detector signalitself. When a pulse is detected, its derivative is calculated. Theinventors have observed that a true pulse caused by the bloodflow from aheartbeat has a certain characteristic. That characteristic is that theratio of the positive peak of the derivative signal to the negative peakof a derivative signal is typically greater than from 1 to 1.4. Motionartifact pulses, on the other hand, have been observed by the inventorsto have approximately a 1:1 ratio of the values of the positive peak tothe negative peak of the derivative signal. Accordingly, a threshold inthe range of 1-1.4 is chosen, with values having a ratio greater thanthe threshold being considered real pulses, while those lower than thethreshold being considered motion artifact pulses.

The present invention provides the advantage of limiting false alarmsdue to motion while still generating an alarm if motion is present foran extended period.

The present invention also provides the advantage of providing a motionindication by analyzing the existing pulse oximeter detector signalwithout requiring additional sensors or hardware.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a housing for a pulse oximeter accordingto the present invention;

FIG. 2 is a block diagram of the electronic circuitry of the pulseoximeter of FIG. 1;

FIG. 3 is a timing diagram illustrating the probationary period of thepresent invention;

FIG. 4 is a state diagram illustrating the normal, motion and noisestate of the present invention;

FIG. 5 is a flowchart illustrating the operation of the presentinvention;

FIG. 6 is a diagram of a typical blood pulse signal and its derivative;and

FIG. 7 is a subroutine flowchart illustrating the motion detection testof FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Housing.

Referring to FIG. 1, the instrument housing 26 of this invention isillustrated. Outwardly, the housing includes a digit display 1,circuitry select button array 2 through 5, alarm status lights 6 through9, an optically coupled adjustment knob 10, sync status light 11, LEDdigital viewmeter 12, and power switch 13. A speaker 15 is placed underand in the instrument housing.

From a connector (not shown) in housing 26 there extend leader wires 27.Wires 27 extend to a detector probe 29. Detector 29 is placed upon thefinger 14 of a patient 28. Utilizing the placement of the detector 29 atthe finger 14, all of the readings in this invention are made possible.

The oximeter housing also includes a motion indicator 30. When lit up,this indicator shows that motion has been detected. In addition, thedigital display 1 will provide a blinking pulse reading with alternatingdashes to indicate that the reliability is suspect due to the detectionof motion.

Oximeter Circuitry.

A description of the electronic circuitry of the Nellcor N-200 pulseoximeter will be first presented, to enable understanding of the presentinvention. This is only an example of one pulse oximeter in which thepresent invention may be used.

Referring to FIG. 2, sensor circuit 100 has red LED 110 and infrared LED120 connected in parallel, anode to cathode, so that the LED drivecurrent alternately illuminates one LED and then the other LED. Circuit100 also includes photodetector 130, preferably a photodiode, whichdetects the level of light transmitted through the patient's tissue,e.g., finger 140, as a single, analog optical signal containing both thered and infrared light plethysmographic, detected optical signalwaveforms.

Patient module 200 includes preamplifier 210 for preamplifying theanalog detected optical signal of photodetector 130. Alternately, thepreamplifier may be in the oximeter itself. Preamplifier 210 may be anoperational amplifier configured as a current to voltage converter,biased by a positive voltage to extend the dynamic range of the system,thereby converting the photocurrent of photodiode 130 into a usablevoltage signal. Patient module 200 also includes leads for passing theLED drive voltages to LEDs 110 and 120.

Saturation analog front end circuit 300 receives the analog opticalsignal from patient module 200 and filters and processes the detectedsignal to provide separate red and infrared analog voltage signalscorresponding to the detected red and infrared optical pulses. Thevoltage signal is passed through low pass filter 310 to remove unwantedhigh frequency components, AC coupled through capacitor 325 to removethe DC component, passed through high pass filter 320 to remove anyunwanted low frequencies and passed through buffer 320 and passedthrough programmable gain stage 330 to amplify and optimize the signallevel presented to synchronous detector 340.

Synchronous detector 340 removes any common mode signals present andsplits the time multiplexed optical signal into two channels, onerepresenting the red voltage signals and the other representing theinfrared voltage signals. Each signal is then passed through respectivefilter chains having two 2-pole 20 hertz low pass filters 350 and 360,and offset amplifier 370 and 380. The filtered voltage signals nowcontain the signal information corresponding to the red and infrareddetected optical signals.

Analog-to-Digital Converter (ADC) 1000 provides the analog to digitalconversions required by the N-200 oximeter. The aforementioned twovoltage signals, the red detected optical signal and the infrareddetected optical signal from patient module 200, are input to ADC 1000.These signals are conventionally multiplexed and digitized by anexpanded range 12-bit analog-to-digital conversion technique, yielding16-bit resolution. The input signals are passed through multiplexor 1010and buffer amplifier 1020. The converter stage includes offset amplifier1030 and programmable gain circuitry 1040 which allows a portion of thesignal to be removed and the remainder to be further amplified forgreater resolution, sample and hold circuit 1050, comparator 1060, and12-bit digital to analog convertor 1080. The buffered signal is passedthrough offset amplifier 1030 to add a DC bias to the signal wherein aportion of the signal is removed and the balance is amplified by beingpassed through programmable gain circuitry 1040 to improve theresolution. The amplified signal is then passed through sample and holdcircuit 1050, the output of which is fed to one input of comparator1060. The other input of comparator 1060 is the output of digital toanalog (DAC) converter 1080 so that when the inputs to comparator 1060are the same, the analog voltage at the sample and hold circuit is giventhe corresponding digital word in DAC converter 1080 which is thenstored in an appropriate memory device as the digitized data for thesample and the next sample is sent to sample and hold circuit 1050 to bedigitized.

DAC 1080 also generates the sensor LED drive voltages, under the controlof microprocessor 2040, using analog multiplexor 610, which separatesthe incoming analog signal into one of two channels for respectivelydriving the red and infrared LEDs, having respective sample and holdcircuits 620 and 630, and LED driver circuit 640 for converting therespective analog voltage signals into the respective positive andnegative bipolar current signals for driving LEDs 110 and 120.

Digital Signal Processor (DSP) 2000 controls all aspects of the signalprocessing operation including the signal input and output andintermediate processing. The apparatus includes 16-bit microprocessor2040 and its associated support circuitry including data bus 10, randomaccess memory (RAM) 2020, read only memory (ROM) 2030, a conventionalLED display device 2020 (not described in detail), and system timingcircuit 2050 for providing the necessary clock synchronizing signals.

Interrupt programs control the collection and digitization of incomingoptical signal data. As particular events occur, various software flagsare raised which transfer operation to various routines that are calledfrom a main loop processing routine.

Probationary Period.

FIG. 3 illustrates a pulse oximeter detector signal 32 with bloodflowpulses being regularly detected in a first period 34. At a time 36, nomore qualified pulses are detected. This cab be due to noise, motionartifact, or the absence of a blood pulse. The oximeter will continuelooking for qualified pulses for a 10 second period 38 after the lastqualified pulse. If no qualified pulse is detected within this time, aprobationary state 40 is entered. In the first portion of theprobationary state, a 6.3 second timer is set for a period 42.

State Diagram.

Referring to FIG. 4 as well, probationary state 40 is indicated in thestate diagram of FIG. 4 as including a motion state 42 and a noise state44. When the 6.3 second timer is set, this is initially the noise state44. The noise state preferrably has affirmative criteria, but can alsosimply be the absence of motion or a qualified signal. An example of anaffirmative criteria is the lack of correlation between the IR and redchannels for a pulse. In the preferred embodiment, at least 2 pulses(noise or otherwise) must be detected as noise in the 6.3 second period.If no pulse is detected for 3.1 seconds, the preceding 3.1 second periodis presumed to be a pulse, and is analyzed accordingly.

If motion is detected, motion state 42 is entered and the 6.3 secondtimer is halted. This can continue for the maximum probation period of50 seconds. If motion continues to be detected after 50 seconds, analarm is generated upon alarm state 46 of FIG. 4 being entered. Thealarm will also be generated when the 6.3 second timer runs out.

Flow Chart of Probationary Period Operation.

FIG. 5 is a flowchart illustrating the software used to implement thestates of FIG. 4. This software would reside in RAM 2020 of FIG. 2.After the start, a startup routine (step A) is entered wherein pulsesare qualified and a good pulse signal is indicated after fourconsecutive qualified pulses. The qualification is done according towell-known techniques. Once this startup is completed, the softwareenters a normal qualification state (step B).

After each pulse is detected, a 10 second timer is started (step C). If10 seconds has passed since the last qualified pulse was detected, thesystem enters the probation state and a 50 second timer is set (step D).The 6.3 second timer is also set (step E). The signal is then monitoredfor the detection of an IR (infrared) pulse (step F). The IR channel ismonitored because the IR tends to be a lower noise source than the redsignal. If no qualified pulse is detected for 6.3 seconds (step G) thealarm is sounded (step H). The 50 second timer is also checked (step I)in case it expires before a particular 6.3 second period.

If an IR pulse is detected in the probation period, a test is then doneto see if motion is detected (step J). The detection of motion is shownin more detail in the motion detection flowchart of FIG. 6. Upon thedetection of motion, the motion display on the monitor is illuminated(step K), and the 6.3 second timer is reset (step E) and the systemcontinues to monitor for another pulse. If a pulse is detected and thereis no motion, a test is done to determine fit should be rejected asbeing noisy (step L). In the noise state, a slower averaging algorithmis used than in the normal qualified state to insure that a falseindication of a good blood pulse is not generated. If there is no noiserejection, it is a qualified pulse, and a qualified pulse count isincremented (step M). Upon the count equalling four consecutive pulses(step N), the probationary period is exited after turning off thedisplay (step O).

Motion Detection.

The motion detection can be from a separate signal, motion sensor, suchas an accelerometer or piezoelectric device attached to the oximetersensor. Preferably, however, motion detection is accomplished byanalyzing the optical detector signal itself.

FIG. 6 illustrates a typical qualified blood pulse. The pulse has arising side 46, and then, after peaking, has a slowly trailing side 48.The derivative of this signal is calculated in microprocessor 2040 ofFIG. 2 of the pulse oximeter. A plot of the derivative is also shown inFIG. 6 for the blood pulse. The derivative has a rising portion 50corresponding to portion 46 of the pulse. After the peak of the pulse,the derivative of the signal rapidly progresses from a positive peak 52to a negative peak 54, and then slowly approaches zero again. The heightof the positive peak of the derivative signal, A, and the height of thenegative peak, B, have been observed to have an A/B ratio of greaterthan 1-1.4 for a typical blood pulse. Motion artifact, on the otherhand, typically has a 1/1 ratio.

Accordingly, the oximeter of the present invention, after determiningthe derivative of the pulse signal, calculates the ratio of A/B andcompares it to a threshold to indicate whether it is a blood pulsesignal or a motion signal pulse. Preferably, the threshold is in a rangeof 1.0-1.4. In a preferred embodiment, a ratio of 260/256 is used, whichequals 1.0196/1. The selection of the exact threshold is tradeoffbetween rejecting good pulses and rejecting motion. A ratio of 1.4 hasbeen observed to reject approximately 95% of motion artifact, but mayalso reject some good pulses as well. The ratio of 1.0196 provides aconservative level to retain most qualified pulses, will giving a goodlevel of rejection of motion artifact.

FIG. 7 is a flowchart for the software routine for determining if apulse is due to motion. When the routine is called, it first determinesthe derivative of the pulse signal (step P). Next, the ratio A/B of thepositive to negative peak of the derivative signal is determined (stepQ). If the ratio is less than 1.0196/1 (step R), the signal ispresumably a motion pulse. If it is greater, it is an indication thatthe pulse is not motion (either qualified or noise pulse) (step S) andthere is a return from the subroutine.

An optional second or alternative motion test may be used in addition toor in place of the ratio test. It has been observed by the inventorsthat for a motion signal, there will be correlation of the pulses in theinfrared and red channels. Noise, on the other hand, tends to beuncorrelated, with different values in the IR and red channels. Thecorrelation of the IR and red pulses are thus determined (optional stepS) and compared (optional step T), and if they are not correlated, thepulse is presumably noise, and there is a return from the subroutine. Ifthe infrared and red pulses are correlated, this is an indication thatthe pulse is due to motion, and a motion flag is set (step U), and thereis a return from the subroutine.

Alternately, a different motion test could be used. For instance, thesaturation value of a pulse could be determined using the ratio ofratios, as is well known in the industry. This calculation can be donefor several different parts of the pulse. For a qualified blood pulse,the values should be approximately the same. If the values differ, itcould be motion or noise. The correlation test could then be run todetermine if it is noise or motion.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. For example, only asingle one of the motion tests of FIG. 8 could be used, or the use of apiezoelectric accelerometer sensor could be substituted for the opticalsignal analysis in order to determine whether a motion signal ispresent. Accordingly, the disclosure of the preferred embodiment of theinvention is intended to be illustrative, without limiting the scope ofthe invention which is set forth in the following claims.

What is claimed is:
 1. A method of operating a pulse oximeter whichemits light, the method comprising the steps of:(a) qualifying pulses ina detector signal, the detector signal corresponding to absorption byblood constituents of the light emitted by the oximeter; (b) upon acondition suggesting a possible alarm situation, setting a first timer,the first timer expiring upon elapse of a time interval of a firstpredetermined duration; (c) if a pulse is detected in the detectorsignal after the first timer has been set and before the first timerexpires, performing the steps of:(c1) determining if the pulse hasresulted from motion artifact; and (c2) if the pulse is determined tohave resulted from motion artifact, resetting the first timer; and (d)if and when the first timer expires, generating an alarm.
 2. The methodof claim 1, wherein step (c2) resets the first timer to expire uponelapse, from when step (c2) is performed, of a time interval of thefirst predetermined duration.
 3. The method of claim 1, wherein thecondition suggesting a possible alarm situation arises upon elapse of atime interval of a predetermined length during which a qualified pulseis not detected.
 4. The method of claim 3, wherein the predeterminedlength is approximately 10 seconds.
 5. The method of claim 1, whereinthe first predetermined duration is approximately 6.3 seconds.
 6. Themethod of claim 1, further comprising the step of:(e) upon the conditionsuggesting a possible alarm situation, setting a second timer, thesecond timer expiring upon elapse of a time interval of a secondpredetermined duration, the second predetermined duration being largerthan the first predetermined duration; and (f) if and when the secondtimer expires, generating the alarm.
 7. The method of claim 6, whereinthe second predetermined duration is approximately 50 seconds.
 8. Themethod of claim 6, further comprising the step of:(g) if a predeterminednumber of qualified pulses are detected before either the first timer orthe second timer expires, stopping the first and second timers.
 9. Themethod of claim 8, wherein the predetermined number is approximately 4.10. The method of claim 1, wherein step (c) further comprises the stepsof:(c3) determining if the pulse has resulted from noise; and (c4) ifthe pulse is determined not to have resulted from noise, resetting thefirst timer.
 11. The method of claim 10, wherein step (c4) resets thefirst timer to expire upon elapse, from when step (c4) is performed, ofa time interval of the first predetermined duration.
 12. A method ofoperating a pulse oximeter which emits light, the method comprising thesteps of:(a) qualifying pulses in a detector signal until a timeinterval of a predetermined length elapses during which a qualifiedpulse is not detected, the detector signal corresponding to absorptionby blood constituents of the light emitted by the oximeter; (b) if andwhen step (a) terminates, setting a count to zero and setting a firsttimer, the first timer expiring upon elapse of a time interval of afirst predetermined duration; (c) after each of steps (b), (e) and (h),setting a second timer, the second timer expiring upon elapse of a timeinterval of a second predetermined duration, the second predeterminedduration being smaller than the first predetermined duration; (d) afterstep (c), if a pulse is detected in the detector signal after the secondtimer has been set in step (c) and before the second timer expires,determining if the pulse has resulted from motion artifact anddetermining if the pulse has resulted from noise; (e) after step (d), ifthe pulse is determined to have resulted from motion artifact, stoppingthe second timer and repeating steps (c) through (h); (f) after step(d), if the pulse is determined to have resulted neither from motionartifact nor from noise, incrementing the count by one, the countindicating a number of qualified pulses detected since step (b) was lastperformed; (g) after step (f), if the count is equal to a predeterminednumber, stopping the first and second timers, and repeating steps (a)through (h); (h) after step (f), if the count is not equal to thepredetermined number, stopping the second timer and repeating steps (c)through (h); and (i) if and when either the first or the second timerexpires, generating an alarm.
 13. The method of claim 12, wherein thepredetermined length is approximately 10 seconds.
 14. The method ofclaim 12, wherein the first predetermined duration is approximately 50seconds.
 15. The method of claim 12, wherein the second predeterminedduration is approximately 6.3 seconds.
 16. The method of claim 12,wherein the predetermined number is approximately
 4. 17. A pulseoximeter comprising:an emitter for emitting light; a photodetector forproducing an analog detector signal corresponding to absorption of saidlight by blood constituents; a circuit, coupled to the photodetector,for producing a digital detector signal corresponding to the analogdetector signal; and a processor, coupled to said circuit and programmedto:(a) qualify pulses in the digital detector signal corresponding toarterial pulses; (b) generate an alarm when no qualified pulses aredetected in a time-out period; and (c) modify the timing of thegeneration of said alarm if the processor determines that motionartifact is present during said time-out period.
 18. The apparatus ofclaim 17, wherein the processor determines whether motion artifact ispresent during said time-out period by analyzing the digital detectorsignal.
 19. The apparatus of claim 17, wherein the processor determineswhether motion artifact is present during said time-out period byanalyzing a signal generated by a motion detector attached to the pulseoximeter.
 20. An apparatus for detecting motion artifact in a digitalpulse oximeter signal comprising:an emitter for emitting light at abody; a photodetector for producing an analog detector signalcorresponding to absorption of said light by the body; a circuit,coupled to the photodetector, for producing the digital pulse oximetersignal from the analog detector signal; and a processor, coupled to saidcircuit and programmed to:(a) detect a pulse waveform in said digitalpulse oximeter signal; (b) determine the derivative of said pulsewaveform; (c) determine the ratio of a positive peak of said derivativeto a negative peak of said derivative; (d) compare said ratio to athreshold value; and (e) indicate motion artifact when said ratio isless than said threshold.